In systems for preventing an incorrect filling, for example for fuel tanks, it is known to provide the filler tube with a permanent magnet and the pump nozzle with a sensor, which releases a supply valve upon coming into contact with the magnetic field of the permanent magnet. In systems of this type, it is known to manufacture the tank from plastic and the filler tube for the tank from aluminum. For this combination of materials in filler tube and tank, the permanent magnet is placed on a plastic jacket between two sealing rings, a plastic cap being placed onto the jacket in such a way that the interior space accommodating the permanent magnet is sealed tight against liquids. This multipart plastic holder is mounted in the aluminum filler tube and is welded to the plastic tank. This design of the filler tube for a system for preventing incorrect filling results in a relatively expensive manufacturing and assembly process because the permanent magnet must be inserted into a separate holder component, which is made up of multiple individual parts.